The contactless power transmission technology is widely used in the field of electronic products due to its convenient and practical features. Currently, the main methods to realize contactless power transmission include magnetic induction and magnetic resonance. The applications of magnetic induction is restricted due to the limitation of transmission distance, while the magnetic resonance wireless power transmission can achieve a long-distance, and high-power energy transmission, which can be widely used for the charging and power supply of electronic terminals, electric vehicles, underwater, and underground electrical equipments.
A typical wireless power transmission device is mainly composed of a power transmitting terminal and a power receiving terminal, both of which can achieve energy transmission through the electromagnetic induction or magnetic resonance. Wherein, a power transmitting terminal includes a transmitter coil and an inverter. The inverter receives DC voltage to produce AC voltage. The transmitter coil receives an alternating voltage to generate alternating magnetic field with frequency of ω0. The receiving coil of the power receiving terminal couples the alternating magnetic field to induce alternating voltage V sin(ω0) with a frequency of ω0. Generally, the lower the frequency of the alternating magnetic field, the shorter the distance power transmission. Therefore, in order to increase the distance of wireless power transmission, it is necessary to enhance the frequency of alternating magnetic field. According to the relevant standard of wireless charging, such as, Qi and PMA wireless charging standard, the frequency of alternating magnetic field is within the range of 100 kHz-500 kH in a electromagnetic induction wireless power transmission system, and the power transmission distance is normally less than one centimeter. According to the A4WP wireless charging standard, if the frequency of an alternating magnetic field is 6.78 MHz in a magnetic resonance wireless power transmission system, the corresponding power transmission distance can be up to several centimeters.
Therefore, in order to enhance the transmission distance, magnetic resonance wireless power transmission mode is normally adopted, and an alternating magnetic field at frequency of 6.78 MHz is generated. Accordingly, the switching device in the inverter should work at the frequency of 6.78 MHz. When the switching device is working at the frequency of 6.78 MHz, the switching device will have a large switching loss, which greatly reduces the life of the switch. In order to reduce the loss, usually a zero switching technology (ZVS) is adopted, to reduce the loss of the switching device.